1. Field of the Invention
The invention relates to stoves and furnaces for space and water heating and more particularly relates to furnaces fired by the combustion of solid, carbonaceous fuels such as wood logs.
2. Brief Description of the Prior Art
The literature is replete with descriptions of space and water heating furnaces (stoves) fired by solid carbonaceous fuels such as wood logs, coal and the like. Lately there has been a renewed interest in such furnaces as a means of heating homes, business establishments, etc. Naturally there is also a renewed interest in increasing the efficiency of such furnaces and reducing their potential for air pollution. The low efficiency (30-50% efficient) and polluting aspects of the heretofore available furnaces is generally related to incomplete combustion and poor heat transfer of fuels. Other furnaces of the prior art are known which exhibit relatively complete combustion, but due to structure, utilize the heat poorly.
Combustion is a chemical reaction releasing heat. The element oxygen is the sole supporter of combustion and therefore combustion may be defined as the combining of a substance with oxygen with consequent release of heat. Perfect combustion is theoretically possible but rare in practice. It comprises bringing each unit of combustible, heated to its ignition temperature, together with the precise quantity of oxygen required for combustion. Three factors are involved in perfect combustion; (1) The precise proportion of oxygen required to combust the fuel (combustible), (2) Thorough and complete admixture of combustible and oxygen and (3) A teperature sufficiently high to promote rapid combination of the oxygen with the combustible fuel.
In practice, the second factor, thorough and complete mixture of proper proportions of oxygen and combustible has been the most difficult to achieve in the heretofore available stoves or furnaces. The principal source of oxygen is air or the atmosphere. In air, there is about 23.15 parts oxygen and 76.85 parts by weight of nitrogen (ignoring minor proportions of other gases). In order to acquire the 1 lb. of oxygen for combustion one must provide at the point of ignition, it is necessary to present at that point 4.32 lbs. of air, including 3.32 lbs. of nitrogen which serves no purpose in combination and in a way is deterimental to desired admixture of the oxygen and combustible. This is because is absorbs heat and reduces the temperature of the products of combustion. The latter is of course detrimental to the third factor described above as an element of perfect combustion. Atmospheric nitrogen is the chief source of heat loss in combustion. It is therefore highly advantageous to provide air to the site of combustion, without an excess beyond that which is required to have the exact amount of oxygen present for complete combustion of the combustible. Excess oxygen, i.e.; that in excess of the proportion which will unite with the combustible, is also undesired because the excess also results in heat loss, absorbed by the excess gas. Further, excess oxygen is accompanied by additional nitrogen which accentuates the heat loss from the combustion. The loss equals the amount of heat units required to heat the excess air to the temperaure of the flue gas and more. Since the specific heat of air is generally higher than the specific heat of the furnace radiators (iron) the excess air absorbs a percentage of heat which otherwise would have been absorbed by the radiators. Excess air will also reduce the percentage of carbon dioxide in the flue gases. With perfect combustion, 12 parts by weight of carbon from the combustible will unit with 32 parts by weight of oxygen. The temperature of this reaction is 4980.degree. F. and the quantity of heat generated is 14,450 B.t.u. per lb. of carbon. The theoretical content of the flue gas is 21 percent by weight carbon dioxide, but this will be reduced in proportion to excess air provided at the combustion site. The percentage of carbon dioxide in the stack gas is an indication of combustion efficiency and a measure of excess air provided at the combustion site. On the other hand, if the oxygen supplied to the site of combustion is inadequate to sustain complete combustion, fuel value is lost due to incomplete heat release. If less oxygen is supplied than is required by the carbon presense, the gases of combustion will pass from the furnace as carbon monoxide (this reaction occurs at 2700.degree. F. and only generates 4350 B.t.u. per lb. of carbon) or as carbon itself in the form of smoke. This is a serious waste of potential fuel.
The furnace of my invention is adapted to permit control of and delivery of preheated air, in advantageous and metered proportions at the combustion site. This is accomplished by delivery of oxygen bearing air in two stages, for intimate admixture during three stages of combustion. The advantage of the furnace of the invention resides in its ability to satisfy the three factors for perfect combustion to a high degree. The result is near perfect combustion with consequent heating efficiency and absence of significant pollutant release to the atmosphere.
For a review of prior art furnaces see Hand, Popular Science, October 1976, page 104 and U.S. Pat. Nos. 420,503; 498,826; 585,027; 661,257; 739,726; 152,566; 1,360,934; and 2,789,554.